Process and materials for imagewise placement of uniform spacers in flat panel displays

ABSTRACT

Process and materials are described for selectively placing uniform spacers on a receptor. Spacer elements are placed on a receptor by selectively irradiating a thermal transfer donor sheet comprising a transferable spacer layer. The transferable spacer layer may include particles or fibers to form a composite. The particles may have a spacing dimension either greater than or less than the thickness of the transferable layer. When the spacing dimension of the particle is greater than the thickness of the transferable layer, then the spacing dimension of the particles control the spacing distance. The process and materials are useful in the manufacture of flat panel displays, particularly, liquid crystal display devices.

FIELD OF THE INVENTION

The present invention relates to a process and materials for placementof spacers onto a receptor which provide uniform spacing and structuralsupport in flat panel displays. More particularly, this inventionrelates to the precise placement of spacers using a thermal transferdonor sheet and an imaging radiation source.

BACKGROUND OF THE ART

Control of the spacings and mechanical forces within the construction ofa flat panel display (i.e., liquid crystal displays, electroluminescentdisplays, vacuum fluorescent displays, field emission displays, andplasma displays) is often critical to the performance of thecorresponding device and depends upon the incorporation of physicalspacers into the corresponding display. For example, in liquid crystaldisplays (LCDs), the polarization of the light exiting the display iscontrolled in part by the optical path length through the liquid crystallayer. In current display technology, the thickness of the liquidcrystal layer is determined by spacers, which may be in the form ofparticles (i.e., spherical beads or fibers), columnar structures (i.e.,posts or pillars), microribs, etc. Spacers have become increasinglyimportant with the desire for light-weight large format displays. Toachieve lighter weight display panels, transparent polymeric substratesare typically used since they are much lighter than glass. However,polymeric substrates are more flexible; thus, requiting a denserpopulation of spacers to maintain a uniform thickness throughout thedisplay panel. The most common and inexpensive method for controllingthe thickness of the liquid crystal layer is to deposit a randomarrangement of particles having a narrow size distribution over theentire surface of the substrate or alignment layer. This process has anobvious disadvantage in that there is no control over the placement ofthe particles resulting in a high percentage of the particles appearingin the display windows, thus decreasing the amount of light that maypass through the display. In many applications, the particles are notanchored to the substrate and may shift or migrate causing artifacts toappear in those areas in the display cell. The spray applicationpresents an additional issue in the manufacturing process. The displayis assembled in Class 10 to 100 cleanrooms to meet the optical qualityrequirements for the liquid crystal displays. Spraying particles onto asurface results in many of the particles becoming airborne, thus makingit difficult to maintain Class 10 to 100 standards. The thinner thelayer desired the smaller the particle required which leads to increasedhandling and application difficulties.

One attempt to overcome the deficiencies in liquid crystal displays asdescribed above is disclosed in U.S. Pat. No. 4,720,173 and JapanesePatent applications, JP 7325298, JP 5203967, and JP 2223922 where aphotoresist material is bonded to the substrate, imaged and developed togenerate spacer entities. This method allows one to more precisely placethe spacer on the substrate; however, the requirement of a developingstep adds an additional step to the process. Liquid development alsoproduces spent developer solutions which must be disposed of. Many ofthe developers contain solvents or have a high pH, thus requiringspecial handling for safety and/or special disposal to meet federal andstate environmental regulations. It is also more difficult to maintain auniform thickness of the spacers when a photoresist is used. Forexample, the developer may etch away more of the surface in one areathan in another.

An alternative approach for controlling spacing in liquid crystaldisplays is described in U.S. Pat. No. 5,268,782; where, amicrostructured substrate is used as both a substrate and a spacerintegrated into one element. To minimize interferences in the windowareas, the microstructured surface typically comprises a series ofparallel ridges (microribs). Even though the percentage of spacerswithin the optical window is minimized, a stripping effect is visible inthe display. Additionally, the deposition of the high viscosity liquidcrystals is more arduous when microribs are used for spacers. Forinstance, it is harder to apply the high viscosity liquid crystalswithout entrapping air which creates an optical defect in the layer.

Clearly there is a need for a method and materials for accurateplacement of structurally supporting spacers which are cost effective,reliable, and eliminate interference with the optical integrity of thedisplay panel.

SUMMARY OF THE INVENTION

The present invention overcomes the deficiencies of the prior art byemploying a method and materials for placement of structurallysupporting spacers on a receptor using a thermal transfer donor sheetand imaging radiation to accurately place uniform spacers in designatedlocations outside the display windows.

The present invention provides a thermal transfer donor sheetcomprising, (a) a support, (b) a transferable spacer layer, and (c) anoptional adhesive layer. At least one of the receptor, support,transferable spacer layer and adhesive layer contains a radiationabsorber which converts a portion of the imaging radiation to heat. Theimaging radiation provides the means for selectively transferring thetransferable spacer layer to a receptor to form spacer elements on thereceptor.

An alternative thermal donor sheet construction is provided whichcomprises; (a) a support, (b) a light-to-heat conversion layercontaining a first radiation absorber, (c) a transferable spacer layer,and (d) an optional adhesive layer. The thermal transfer donor sheet mayoptionally include a non-transferable interlayer interposed between thelight-to-heat conversion layer and the transferable spacer layer. Asecond radiation absorber may be present in the receptor, support,non-transferable interlayer, transferable spacer layer or adhesivelayer.

The transferable spacer layer may be either a non-composite organicmaterial or a composite containing particles having spacing dimensionswhich are either smaller than or larger than the thickness of thetransferable spacer layer. When the spacing dimensions of the particlesare smaller than or equal to the thickness of the transferable spacerlayer, then the thickness of the transferable layer controls the spacingdistance between the receptor and an additional substrate attached tothe spacer elements in forming a flat panel display device. When thespacing dimensions of the particles are greater than the thickness ofthe transferable spacer layer, then the spacing dimensions of theparticles control the spacing distance within the flat panel display.

In another embodiment, a process is described for selectively placingspacer elements on a receptor for use in a flat panel display includingthe steps of: (1) providing the thermal transfer donor sheet describedabove, (2) placing in intimate contact the receptor with thetransferable spacer layer of the thermal transfer donor sheet, (3)irradiating at least one of the thermal transfer donor sheet or thereceptor in an imagewise pattern with imaging radiation such that theradiation absorber in either the receptor or thermal transfer donorsheet construction absorbs a portion of the imaging radiation andconverts that radiation to heat, (4) transferring the transferablespacer layer in the irradiated areas to the receptor, and (5) removingthe thermal transfer donor sheet to form spacer elements correspondingto the irradiated areas on the receptor.

In yet another embodiment, a process is described for use inconstructing a liquid crystal display device wherein the above describedprocess further includes the steps of (6) attaching the spacer elementsto a substrate to form cavities between the substrate and the receptor,(7) filling the cavities with liquid crystal materials, and (8) sealingthe periphery of the substrate to the receptor.

As used herein the phrase "in intimate contact" refers to sufficientcontact between two surfaces such that the transfer of materials may beaccomplished during the imaging process to provide sufficient transferof material within the thermally addressed areas. In other words, noimperfections are present in the imaged areas which render the articlenon-functional.

"Spacers" or "spacer elements" refer to elements which provide a meansof separating two parallel substrates (or supports) and may also providestructural support for one or both of the same two parallel substrates.

"Spacing dimension" refers to the spacing distance between two parallelsubstrates provided by the spacer elements. For those spacer elementsbased on a non-composite organic material or a composite materialwherein the composite contains particles which are smaller than thethickness of the transferable spacer layer, the spacing dimension isequal to the thickness of the spacer layer. However, when the compositecontains particles having spacing dimensions that are greater than thethickness of the transferable spacer layer, the spacing dimension isequal to the diameter or height of the particles as orientedperpendicular to the substrates. In other words, if the particles arespherical in shape, then the diameter of the sphere is the dimensionmeasured. If the particles are cylindrical in shape (i.e., rods), thenthe diameter of the cylinder is used if the cylindrical particles areoriented such that the circular dimension is perpendicular to thesubstrate. However, when the cylindrical shaped particles are orientedsuch that the length of the cylindrical particles are perpendicular tothe substrates (i.e., pillar between the substrates), then the height ofthe cylinder is used as the spacing dimension.

"Imaging radiation" refers to energy from a radiation source that cancause an image-wise transfer of a mass transfer layer from a thermaltransfer donor sheet to a receptor (or substrate).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a process for placing spacer elementson a receptor (or substrate) for use in a flat panel display. The spacerelements are placed on the receptor by selectively irradiating a thermaltransfer donor sheet comprising, in order: (a) a support, (b) anoptional light-to-heat conversion layer, (c) an optionalnon-transferable interlayer, (d) a transferable spacer layer and (e) anoptional adhesive layer. The process includes the following steps: (i)placing in intimate contact a receptor and the thermal transfer donorsheet described above, (ii) irradiating at least one of the thermaltransfer donor sheet or the receptor (or a portion thereof, i.e.,substrate, spacer layer, interlayer, light-to-heat conversion layer,and/or adhesive layer) with imaging radiation to provide sufficient heatin the irradiated areas to transfer the spacer layer to the receptor,and (iii) transferring the transferable spacer layer in the irradiatedareas to the receptor.

The thermal transfer donor sheet of the present invention can beprepared by depositing layers (b), (c), (d) and/or (e) described aboveonto a support. The support may be constructed of any material known tobe useful as a support for a thermal transfer donor sheet. The supportmay be either a rigid sheet material such as glass or a flexible film.The support may be smooth or rough, transparent, opaque, translucent,sheet-like or non-sheet-like. Suitable film supports include polyesters,especially polyethylene terephthalate (PET), polyethylene naphthalate(PEN), polysulfones, polystyrenes, polycarbonates, polyimides,polyamides, cellulose esters such as, cellulose acetate and cellulosebutyrate, polyvinyl chlorides and derivatives thereof, and copolymerscomprising one or more of the above materials. Typical thicknesses ofthe support are between about 1 to 200 microns.

The transferable spacer layer may include organic materials oralternatively a composite comprising organic materials havingincorporated therein particles or fibers. Suitable materials include anynumber of known polymers, copolymers, oligomers and/or monomers.Suitable polymeric binders include materials such as thermoset,thermosettable, or thermoplastic polymers, including phenolic resins(i.e., novolak and resole resins), polyvinylacetates, polyvinylidenechlorides, polyacrylates, cellulose ethers and esters, nitrocelluloses,polycarbonates, polysulfones, polyesters, styrene/acrylonitrilepolymers, polystyrenes, cellulose ethers and esters, polyacetals,(meth)acrylate polymers, polyvinylidene chloride, α-chloroacrylonitrile,maleic acid resins and copolymers, polyimides, poly(amic acids), andpoly(amic esters) and mixtures thereof.

When the transferable spacer layer includes a thermosettable binder, thethermosettable binder may be crosslinked after transfer to the receptor.The binder may be crosslinked by any method which is appropriate forthat particular thermosettable binder, for example, exposing thethermosettable binder to heat, irradiating with a suitable radiationsource, or a chemical curative.

Particles or fibers may be added to the transferable spacer layer toform a composite. The addition of particles or fibers to thetransferable spacer layer may be accomplished by using any knownparticle or fiber with a spacing dimension less than or equal to thespacing required in the particular display device of interest. Theparticles may have a spacing dimension smaller than the thickness of thetransferable spacer layer or a spacing dimension larger than thethickness of the transferable spacer layer. When the particle size issmaller, the thickness of the transferable spacer layer controls thespacing within the display device. Whereas, when larger particles areused the spacing dimension of the particles used in the compositecontrols the spacing in the display device. Preferably at least 5% ofthe particles have a spacing dimension greater than the thickness of thespacer layer and more preferably at least 10%. Either approach may beused as a means for achieving uniform separation and support of thesubstrates within the display. Suitable particles include organic and/orinorganic materials (solid or hollow) having any suitable shape (i.e.,spheres, rods, posts, triangles, and trapezoids) and size distributionconsistent with maintaining the desired separation. Preferred particlesinclude current LCD spacer spheres, rods, etc. comprised of glass orplastic such as those referenced in Japanese Kokai Patent ApplicationNo. HEI 7 1995!-28068; U.S. Pat. Nos. 4,874,461; 4,983,429; and5,389,288. In LCD displays, it is preferred that the standard deviationfor the size distribution of particles is + or -20% of the mean particlespacing dimension (i.e., mean diameter of a spherical or cylindricalshaped particle, or average height of a cylindrical shaped particle).More preferably, the standard deviation is + or -10% of the mean. Mostpreferably, the standard deviation is + or -5% of the mean. When a fiberis used, the dimensions are typically measured as the denier (orfineness) of the fiber. The length of the fiber is preferably less thanthe diameter of the transferred spacer element.

Dispersants, surfactants and other additives (i.e., antioxidants, lightstabilizers, and coating aides) may be included to aide in thedispersion of the particles and/or fibers or impart other desirableproperties to the transferable spacer layer as known to those skilled inthe art.

The compressibility of the element bearing the forces in the display(e.g., the particles in the case where the spacer layer comprisesparticles with a particle spacing dimension greater than the thicknessof the transferable spacer layer and the transferable spacer layer incases where the spacer layer does not comprise particles with particlespacing dimensions greater than the thickness of the transferable spacerlayer) should be sufficient to maintain a uniform spacing gap in thecorresponding display.

The thermal transfer donor sheet may also include other ingredientsknown to be useful with mass transfer donor sheets, such as radiation(or light) absorbing materials that absorb the imaging radiation andconverts that radiation energy into heat energy, thus facilitatingtransfer of the transferable spacer layer from the donor sheet to areceptor. The radiation absorbing material may be any material known inthe art that absorbs a portion of the incident imaging radiation andconverts that imaging radiation energy to heat energy. Suitableradiation absorbing materials include absorbing dyes (i.e., dyes thatabsorb light in the ultraviolet, infrared, or visible wavelengths),binders or other polymeric materials, organic or inorganic pigments thatcan be a black-body or non-black-body absorber, metals or metal films,or other suitable absorbing materials.

Examples of radiation absorbing materials that have been found to beparticularly useful are infrared absorbing dyes. Descriptions of thisclass of dyes may be found in Matsuoka, M., Infrared AbsorbingMaterials, Plenum Press, New York, 1990, in Matsuoka, M., AbsorptionSpectra of Dyes for Diode Lasers, Bunshin Publishing Co., Tokyo, 1990,in U.S. Pat. Nos. 4,772,583; 4,833,124; 4,912,083; 4,942,141; 4,948,776;4,948,777; 4,948,778; 4,950,639; 4,940,640; 4,952,552; 5,023,229;5,024,990; 5,286,604; 5,340,699; 5,401,607 and in European Patent Nos.321,923 and 568,993. Additional dyes are described in Bello, K. A. etal., J. Chem. Soc., Chem. Commun., 452 (1993) and U.S. Pat. No.5,360,694. IR absorbers marketed by American Cyanamid or GlendaleProtective Technologies under the designation IR-99, IR-126 and IR-165may also be used, as disclosed in U.S. Pat. No. 5,156,938. In additionto conventional dyes, U.S. Pat. No. 5,351,617 describes the use ofIR-absorbing conductive polymers as radiation absorbing materials.

Other examples of preferred radiation absorbing materials includeorganic and inorganic absorbing materials such as carbon black, metals,metal oxides, or metal sulfides. Representative metals include thosemetallic elements of Groups Ib, IIb, IIIa, IVa, IVb, Va, Vb, VIa, VIband VIII of the Periodic Table, as well as alloys thereof, or alloysthereof with elements of Groups Ia, IIa, and IIIb, or mixtures thereof.Particularly preferred metals include Al, Bi, Sn, In or Zn, and alloysthereof or alloys thereof with elements of Groups Ia, IIa and IIIb ofthe Periodic Table, or compounds or mixtures thereof. Suitable compoundsof these metals include metal oxides and sulfides of Al, Bi, Sn, In, Zn,Ti, Cr, Mo, W, Co, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zr and Te, and mixturesthereof.

The radiation absorbing material may be present in the thermal transferdonor sheet as a separate layer, commonly referred to as a "light toheat conversion layer" (LTHC), interposed between the support and thetransferable spacer layer. A typical light to heat conversion layerincludes one or more layers of organic or inorganic materials that arecapable of absorbing the imaging radiation and are preferably thermallystable. It is also desirable that the light to heat conversion layerremain substantially intact during the imaging process. When a metallicfilm is used for the light to heat conversion layer, the metallic layerpreferably has a thickness between 0.001 to 10 μm, more preferablybetween 0.002 to 1.0 μm.

Alternatively, a light to heat conversion layer may consist of lightabsorbing particles (i.e., carbon black) dispersed in a binder. Suitablebinders include film-forming polymers such as thermoset, thermosettable,or thermoplastic polymers, such as phenolic resins (i.e., novolak andresole resins), polyvinylacetates, polyvinylidene chlorides,polyacrylates, cellulose ethers and esters, nitrocelluloses,polycarbonates, and mixtures thereof. When this type of light to heatconversion layer is used, the dry coating thickness is preferablybetween 0.05 to 5.0 micrometers (μm), more preferably 0.1 to 2.0 μm.

When the LTHC layer is present, an optional non-transferable interlayermay be interposed between the transferable spacer layer and the LTHClayer. The incorporation of a interlayer reduces the level ofcontamination of the resulting transferred image from the light-to-heatconversion layer and decreases the amount of distortion in thetransferred image. The interlayer may be either an organic or inorganicmaterial. To minimize damage and contamination of the transferred spacerelement, the interlayer is preferably a continuous coating which has ahigh thermal resistance and remains substantially intact and in contactwith the LTHC layer during the imaging process. Suitable organicmaterials include both thermoset (crosslinked) and thermoplasticmaterials. The interlayer may be either transmissive or reflective atthe imaging radiation wavelength output.

Suitable thermoset resins useful in the interlayer include both thermal-and radiation-crosslinked materials, such as crosslinkedpoly(meth)acrylates, polyesters, epoxies, and polyurethanes. For ease ofapplication, the thermoset materials are usually coated onto thelight-to-heat conversion layer as thermoplastic precursors andsubsequently crosslinked to form the desired crosslinked interlayer.Classes of suitable thermoplastic materials include polysulfones,polyesters, and polyimides. The thermoplastic interlayer may be appliedto the light-to-heat conversion layer using conventional coatingtechniques (i.e., solvent coating, spray coating, or extrusion coating).The optimum thickness of the interlayer is determined by the minimumthickness at which transfer of the light-to-heat conversion layer anddistortion of the transferred spacer layer are eliminated, typicallybetween 0.05 μm and 10 μm.

Suitable inorganic materials for use as interlayer materials includemetals, metal oxides, metal sulfides, and inorganic carbon coatings,which are highly transmissive at the imaging radiation wavelength andmay be applied to the light-to-heat-conversion layer using conventionaltechniques (i.e., vacuum sputtering, vacuum evaporation, or plasma jet).The optimum thickness is determined by the minimum thickness at whichtransfer of the light-to-heat conversion layer and distortion of thetransferred layer are eliminated, typically between 0.01 μm and 10 μm.

The thermal transfer donor sheet may include an optional adhesive layerovercoated on the surface of the transferable spacer layer. The adhesivelayer provides improved transfer of the transferable spacer layer to areceptor by means of a thermally activated adhesive. The adhesivetopcoat is preferably colorless; however, in some applications atranslucent or opaque adhesive may be desirable to enhance the contrastof the display or to provide special effects. The adhesive layer ispreferably non-tacky at room temperature. The adhesive layer may alsoinclude a light absorbing material to further assist the transferefficiency of the image. Preferred adhesives include thermoplasticmaterials having melting temperatures between approximately 30° C. and110° C. Suitable thermoplastic adhesives include materials such aspolyamides, polyacrylates, polyesters, polyurethanes, polyolefins,polystyrenes, polyvinyl resins, copolymers and combination thereof. Theadhesive may also include thermal or photochemical crosslinkers toprovide thermal stability and solvent resistance to the transferredimage. Crosslinkers include monomers, oligomers and polymers which maybe crosslinked thermally or photochemically by either external initiatorsystems or internal self-initiating groups. Thermal crosslinkers includematerials capable of crosslinking when subjected to thermal energy.

Alternatively, radiation absorbing materials may be incorporated intothe receptor, or in a separate topcoat deposited on the surface of thereceptor (i.e., a black matrix on the receptor, an adhesive topcoatdeposited on the surface of the receptor) to assist in the transfer ofthe spacer layer to the receptor. If the radiation absorbing material ispresent in the receptor, or is in a portion of the thermal transferdonor sheet that is transferred to the receptor during imaging process,then it is preferred that the radiation absorbing material not interferewith the performance properties (i.e., the desired optical properties)of the imaged receptor.

The receptor may be any flat panel display element benefiting from theapplication of spacers. The spacers are precisely placed in the desiredlocations to avoid optical interference in the display windows of thedisplay device. The receptor may be optionally coated with an adhesivetopcoat to facilitate the transfer of the transferable spacer layer tothe receptor. The receptor may also have deposited on the surface ablack matrix to enhance viewing contrast. The black matrix may be formedby deposition of inorganic (i.e., metal and/or metal oxides, and metalsulfides) or organic materials (i.e., dyes in an organic binder) or acombination of both (i.e., carbon black dispersed in a binder). Theblack matrices generally have a thickness between 0.005 to 5 microns.Typically, the receptor has a thickness between 1 to 2000 microns.

In the practice of the present invention, the thermal imaging element ispositioned such that upon application of the imaging radiation (orlight), the LTHC layer absorbs the imaging radiation and converts it toheat in the irradiated areas which in turn promotes the transfer of thetransferable spacer layer in the irradiated areas to form the spacerelements on the receptor.

The formation of the spacers may be effected by appropriate modulationof a imaging radiation source or by exposure through a mask. The spacersmay be precisely placed in the desired locations to avoid opticalinterference in the display windows of the display device. A variety oflight-emitting sources can be utilized in the present inventionincluding flash lamps, high powered gas lasers, infrared, visible, andultraviolet lasers. In an analog system, a mask is used to selectivelyfilter the radiation in an imagewise pattern corresponding to thedesired spacer locations. Flash lamps having sufficient energy output totransfer the spacer layer may be used in the analog systems. In adigitally addressed system, a laser or laser diode is typically used toimagewise transfer the spacer layer onto the substrate in the desiredspacer locations. Preferred lasers for use in this invention includehigh power (>100 mW) single mode laser diodes, fiber-coupled laserdiodes, and diode-pumped solid state lasers (i.e., Nd:YAG and Nd:YLF),and the most preferred lasers are diode-pumped solid state lasers. Inboth the analog and digitally addressed systems, the spacers may beprecisely placed in the desired locations to avoid optical interferencein the display windows of the display device. Since the spacers areselectively transferred from the thermal transfer element onto thesubstrate, no liquid process steps are necessary to develop the image.The direct imaging process eliminates the need for additional equipment,additional process steps to develop the image and disposal of spentdevelopers.

During laser exposure it may be desirable to minimize formation ofinterference pattern due to multiple reflections from the imagedmaterial. This can be accomplished by various methods. The most commonmethod is to effectively roughen the surface of the thermally imageableelement on the scale of the incident imaging radiation as described inU.S. Pat. No. 5,089,372. An alternate method is to employ the use of ananti-reflection coating on the second interface that the incidentillumination encounters. The use of anti-reflection coatings is wellknown in the art, and may consist of quarter-wave thicknesses of acoating such as magnesium fluoride, as described in U.S. Pat. No.5,171,650. Due to cost and manufacturing constraints, the surfaceroughening approach is preferred in many applications.

A representative application of the process for using the thermaltransfer donor sheet described herein for selective placement of spacerson a substrate is in the manufacture of liquid crystal display devices.A twisted nematic display device is an example of a typical liquidcrystal display, which comprises a cell or envelope formed by placing apair of transparent, planar substrates, in register, overlying andspaced apart from one another using spacer elements. The periphery ofthe substrates are joined and sealed with an adhesive sealant usuallyapplied by a screen printing technique to provide an enclosed cell. Theshallow space or cavity between the spacer elements on the substrates isfilled with liquid crystal materials just prior to final sealing.Conductive, transparent electrodes are arranged on the inside surface ofthe substrates in either a segmented or X-Y matrix design to form aplurality of picture elements. Alignment coatings are applied toportions of the interior surface of the liquid crystal display cell tocause a desired orientation of the liquid crystal material at itsinterface with the surface of the display. This ensures that the liquidcrystal rotates light through angles which are complementary to thealignment of the polarizers associated with the cell. Polarizingelements are optional depending on the type of display and may beassociated with one or more surfaces of the display when used. Areflector element may be associated with the bottom substrate when areflective rather an a transmissive display is desired. In that event,the bottom substrate may not have to be transparent. The receptor mayoptionally contain an alignment layer coated on the surface, in whichcase, the spacers are applied to the alignment layer. The spacers areplaced on the receptor (or alignment layer) using the process previouslydescribed by selectively irradiating the thermal transfer donor elementin intimate contact with the receptor.

The components and assembly techniques of liquid crystal displays asdescribed above are well known. For example, general details forassembly may be found in "Materials and Assembling Process of LCDs"Liquid Crystals--Applications and Uses, Bitendra Bahadur, Ed., WorldScientific Publishing Co. Pte. Ltd., Volume 1, Chapter 7 (1990).

The following non-limiting examples further illustrate the presentinvention.

EXAMPLES

The materials employed below were obtained from Aldrich Chemical Co.(Milwaukee, Wis.) unless otherwise specified.

The following Examples illustrate the formation of spacers on a glasssubstrate using the following process. The spacers were formed on aglass substrate by placing the coated side of the thermal transfer donorelement in intimate contact with the glass substrate in a recessedvacuum frame and then imaged using a single mode Nd:YAG laser in a flatfield scanning configuration. The laser was incident upon the substrateside of the thermal transfer element and normal to the transferelement/glass receptor surface. Scanning was done with a lineargalvonometer focused on to the image plane using an f-theta scan lens.The power on the image plane was 8 watts and the laser spot size(measured at the l/e² intensity) was 140×150 microns. The linear laserspot velocity was 4.6 meters/second measured at the image plane.

Example 1

A carbon black light-to-heat conversion layer was prepared by coatingthe following LTHC Coating Solution 1 onto a 0.1 mm (3.88 mil) PETsubstrate with a #9 coating rod.

    ______________________________________                                        LTHC Coating Solution 1:                                                      Component              Parts by Weight                                        ______________________________________                                        Raven ™ 760 Ultra carbon black pigment                                                            3.78                                                   (available from Columbian Chemicals,                                          Atlanta, GA)                                                                  Butvar ™ B-98 (polyvinyl butyral resin, available                                                 0.67                                                   from Monsanto, St. Louis, MO)                                                 Joncryl ™ 67 (acrylic resin, available from S. C.                                                 2.02                                                   Johnson & Son, Racine, WI)                                                    Disperbyk ™ 161 (dispersing aid, available from                                                   0.34                                                   Byk Chemie, Wallingford, CT)                                                  FC-430 (fluorochemical surfactant, available                                                         0.01                                                   from 3M, St. Paul, MN)                                                        SR 454 (pentaerythrito1 tetraacrylate available                                                      22.74                                                  from Sartomer, Exton, PA)                                                     Duracure ™ 1173 (2-hydroxy-2 methyl-1-phenyl-                                                     1.48                                                   1-propanone photoinitiator, available from                                    Ciba-Geigy, Hawthorne, NY)                                                    1-Methoxy-2-propanol   27.59                                                  Methyl ethyl ketone    41.38                                                  ______________________________________                                    

The coating was dried at 80° C. for 3 minutes and subsequently UV-curedon a Fusion UV Curing Model MC-6RQN fitted with 300 w/inch H-bulbs andutilizing a web transport speed of 22.9 m/min. (75 ft./min.) The curedcoating had thickness of 3 microns and an optical density of 1.2 at 1064nm.

Onto the carbon black coating of the light-to-heat conversion layer theProtective Interlayer Solution 1 was coated using a #4 coating rod.

    ______________________________________                                        Protective Interlayer Coating Solution 1:                                     Component               Parts by Weight                                       ______________________________________                                        Neorad ™ NR-440 (50% nonvolatiles in water,                                                        38.00                                                 available from Zeneca Resins, Wilmington,                                     MA)                                                                           Duracure ™ 1173      1.00                                                  Water                   61.00                                                 ______________________________________                                    

The coating was dried at 80° C. for 3 minutes and subsequently UV-curedon a Fusion UV Curing Model MC-6RQN fitted with 300 w/inch H-bulbs andutilizing a web transport speed of 22.9 m/min. (75 ft/min.). The curedcoating had thickness of 1 micron.

The interlayer was then overcoated with Transferable Spacer LayerCoating Solution 1 provided below:

    ______________________________________                                        Transferable Spacer Layer Coating Solution 1:                                 Component               Parts by Weight                                       ______________________________________                                        Elvacite ™ 2776 (acrylic resin, available from                                                     20.00                                                 ICI Acrylics, St. Louis, MO)                                                  N,N-dimethylethanolamine                                                                              76.00                                                 Water                   4.00                                                  ______________________________________                                    

Four separate coatings were made using #4, #6, #8 and #10 wire woundbars and all coatings were dried at 60° C. for 3 minutes. Thethicknesses of the dried coatings on the four resultant samples rangedfrom 1 to 2 microns.

The thermal transfer elements were imaged onto 75 mm×50 mm×1 mm glassslides using the laser imaging system described above. The spacer layerswere successfully transferred to the glass to give parallel linesapproximately 95 microns wide. It was also demonstrated that thethickness of the transferred spacers can be increased by transferringadditional spacer layers onto previously transferred spacers to createspacer lines with heights many times the height of the originaltransferred spacer lines. This was accomplished by repeating the imagingstep with additional thermal transfer elements with the positions of thetransferring lines registered to the positions of the previouslytransferred spacers.

Example 2

This example illustrates a thermal transfer element having a compositetransferable spacer layer containing silica particles with particlespacing dimensions smaller than the thickness of the spacer transferlayer.

A carbon black light-to-heat conversion layer was prepared by coatingthe following LTHC Coating Solution 2 onto a 0.1 mm (3.88 mil) PETsubstrate with a Yasui Seiki Lab Coater, Model CAG-150 using amicrogravure roll of 228.6 helical cells per lineal cm (90 helical cellsper lineal inch).

    ______________________________________                                        LTHC Coating Solution 2:                                                      Component              Parts by Weight                                        ______________________________________                                        Raven ™ 760 Carbon Black pigment                                                                  3.78                                                   Butvar ™ B-98       0.67                                                   Joncryl ™ 67        2.02                                                   Disperbyk ™ 161     0.34                                                   FC-430                 0.01                                                   SR 351 (trimethylolpropane triacrylate, available                                                    22.74                                                  from Sartomer, Exton, PA)                                                     Duracure ™ 1173     1.48                                                   1-Methoxy-2-propanol   27.59                                                  Methyl ethyl ketone    41.38                                                  ______________________________________                                    

The coating was in-line dried at 40° C. and UV-cured at 6.1 m/min. (20ft./min.) using a Fusion Systems Model I600 (400 watts/inch) UV curingsystem fitted with H-bulbs. The dried coating had a thicknessapproximately 3.5 microns and an optical density of 1.2 at 1064 nm.

Onto the carbon black coating of the light-to-heat conversion layer wasrotogravure coated Protective Interlayer Coating Solution 2 using theYasui Seiki Lab Coater, Model CAG-150. This coating was in-line dried(40° C.) and UV-cured at 6.1 m/min. (20 ft/min.) using a Fusion SystemsModel I600 (600 watts/inch) UV-curing system fitted with H-bulbs. Thethickness of the resultant interlayer coating was approximately 1 μm.This LITI donor element was denoted as "LITI Donor Element I".

    ______________________________________                                        Protective Interlayer Coating Solution 2:                                     Component       Parts by Weight                                               ______________________________________                                        Butvar ™ B-98                                                                              0.99                                                          Joncryl ™ 67 2.97                                                          SR-351          15.84                                                         Daracure ™ 1173                                                                            0.99                                                          1-Methoxy-2-propanol                                                                          31.68                                                         2-Butanone      47.52                                                         ______________________________________                                    

The protective interlayer of LITI Donor Element I was then overcoatedwith the following Transferable Spacer Layer Coating Solution 2 using a#10 wire wound bar and dried at 60° C. for 2 minutes. The thickness ofthe dried coating was determined by profilometry to be approximately 2.7microns.

    ______________________________________                                        Transferable Spacer Layer Coating Solution 2:                                 Component              Parts by Weight                                        ______________________________________                                        Elvacite ™ 2776     9.62                                                   EMS-American Grilon Primid XL-552 (available                                                         0.39                                                   from EMS-American Grilon, Sumter, SC)                                         Nalco Chemical 2327 (40 weight % SiO.sub.2 in                                                        25.00                                                  water, available from Nalco Chemicals,                                        Chicago, IL)                                                                  N,N-dimethylethanolamine                                                                             3.96                                                   Water                  76.04                                                  ______________________________________                                    

The spacer layer (organic binder/SiO₂ coating) of the thermal transferelement was placed in intimate contact with a 75 mm×50 mm×1 mm glassslide receptor and imaged in an imagewise fashion using the proceduredescribed above to transfer spacer lines approximately 60 microns wideand 2.7 microns thick with a center-to-center spacing of 400 microns.After imaging, the imaged glass receptor was heated to 250° C. in anitrogen atmosphere for 1 hour to crosslink the spacer lines.

Example 3

This example illustrates a thermal transfer element having a compositetransferable spacer layer containing particles having a spacingdimension greater than the thickness of the transferable spacer layer.

The protective interlayer of LITI Donor Element I in Example 2 wasovercoated with Transferable Spacer Layer Coating Solution 3 using thesame procedure as described in Example 2 for coating Transferable SpacerLayer Coating Solution 2.

    ______________________________________                                        Transferable Spacer Layer Coating Solution 3:                                 Component            Parts by Weight                                          ______________________________________                                        Elvacite ™ 2776   14.42                                                    EMS-American Grilon Primid XL-552                                                                  0.58                                                     ZrO.sub.2 4-8 micron diameter particles*                                                           5.00                                                     N,N-dimethylethanolamine                                                                           4.00                                                     Water                76.00                                                    ______________________________________                                         *As prepared in preparation A, Example 5 of U.S. Pat. No. 5,015,373      

The spacer layer (organic binder/ZrO₂ coating) of the thermal transferelement was placed in intimate contact with a 75 mm×50 mm×1 mm glassslide receptor and imaged in an imagewise fashion using the proceduredescribed above to transfer spacer lines approximately 105 microns wideand 3.0 microns thick with a center-to-center spacing of 300 microns.After imaging, the imaged glass receptor was heated to 250° C. in anitrogen atmosphere for 1 hour to crosslink the spacer lines.

What is claimed:
 1. A process for selectively placing spacers on areceptor for use in a flat panel display comprising the steps of:(i)providing a receptor and a thermal transfer donor sheet, said donorsheet comprising in order,(a) a support, (b) a transferable spacerlayer, and (c) an optional adhesive layer,wherein at least one of saidreceptor, said support, said spacer layer or said optional adhesivelayer comprises a radiation absorber; (ii) placing in intimate contactsaid receptor with said transferable spacer layer of said thermaltransfer donor sheet; (iii) irradiating at least one of said thermaltransfer donor sheet or said receptor in an imagewise pattern withimaging radiation, said imaging radiation being absorbed by saidradiation absorber and converted to sufficient heat to transferirradiated areas of said transferable spacer layer of said thermaltransfer donor sheet to said receptor; (iv) transferring saidtransferable spacer layer in said irradiated areas to said receptor; and(v) removing said thermal transfer donor sheet to form spacer elementscorresponding to said irradiated areas on said receptor.
 2. A processfor selectively placing spacers on a receptor for use in a flat paneldisplay comprising the steps of:(i) providing a receptor having a firstsurface and a second surface and a thermal transfer donor sheet, saiddonor sheet comprising in order,(a) a support, (b) a light to heatconversion layer comprising a first radiation absorber, (c) atransferable spacer layer, and (d) an optional adhesive layer; (ii)placing in intimate contact said first surface of said receptor withsaid transferable spacer layer of said thermal transfer donor sheet;(iii) irradiating at least one of said thermal transfer donor sheet orsaid receptor in an imagewise pattern with imaging radiation, saidimaging radiation being absorbed by said first radiation absorber andconverted to sufficient heat to transfer irradiated areas of saidtransferable spacer layer of said thermal transfer donor sheet to saidfirst surface of said receptor; (iv) transferring said transferablespacer layer in said irradiated areas to said first surface of saidreceptor; and (v) removing said thermal transfer donor sheet to formspacer elements corresponding to said irradiated areas on said firstsurface of said receptor.
 3. The process of claim 2 wherein at least oneof said receptor, said support, said transferable spacer layer or saidoptional adhesive layer comprises a second radiation absorber whichabsorbs said imaging radiation.
 4. The process of claim 2 furthercomprising a non-transferable interlayer interposed between said lightto heat conversion layer and said transferable spacer layer of saidthermal transfer donor sheet.
 5. The process of claim 4 wherein at leastone of said receptor, said support, said non-transferable interlayer orsaid transferable spacer layer comprises a second radiation absorberwhich absorbs said imaging radiation and converts said radiation toheat.
 6. The process of claim 2 wherein said receptor further comprisesan adhesive topcoat deposited on said first surface.
 7. The process ofclaim 2 wherein said transferable spacer layer is a composite comprisingparticles having spacing dimensions less than the thickness of saidspacer layer.
 8. The process of claim 2 wherein said transferable spacerlayer is a composite comprising particles having a mean spacingdimension greater than the thickness of said spacer layer.
 9. Theprocess of claim 8 wherein the size distribution of said particles has astandard deviation of + or -20% of said mean spacing dimension of saidparticles.
 10. The process of claim 8 wherein the size distribution ofsaid particles has a standard deviation of + or -10% of said meanspacing dimension of said particles.
 11. The process of claim 8 whereinthe size distribution of said particles has a standard deviation of + or-5% of said mean spacing dimension of said particles.
 12. The process ofclaim 2 wherein said transferable spacer layer is a composite comprisingparticles wherein at least 5% of said particles have a spacing dimensiongreater than the thickness of said spacer layer.
 13. The process ofclaim 2 wherein said transferable spacer layer is a composite comprisingparticles wherein at least 10% of said particles have a spacingdimension greater than the thickness of said spacer layer.
 14. Theprocess of claim 2 further comprising the steps of:(vi) attaching saidspacer elements to a substrate to form cavities between said substrateand said receptor; (vii) filling said cavities with liquid crystalmaterials; and (viii) sealing the periphery of said substrate to saidreceptor.